1. Field of the Invention
The present invention relates generally to a cooling system for a passenger compartment or cabin of a vehicle and more particularly to a cabin cooling system that captures and exhausts boundary layers of hot air that form on interior surfaces of vehicle windows that absorb energy and become hot from incident solar radiation.
2. Description of Related Art
An ongoing concern for the transportation industry is heat build-up within passenger compartments, i.e., cabins, of vehicles parked in the sun for long periods of time. The term "vehicles" includes, but is not limited to, automobiles (i.e., passenger vehicles), light duty vehicles such as sport utility vehicles (SUVs), trucks, and minivans, and miscellaneous other vehicles such as buses, larger trucks, tractors, trains, airplanes, and the like. Heat build-up leads to interior air temperatures up to 160.degree. F. and surface temperatures up to 250.degree. F. (i.e., "soak temperatures") as windows provide a greenhouse-type effect in enclosed vehicles. These higher temperatures cause passenger discomfort and damage heat sensitive equipment and materials, and also, result in higher peak cooling loads that must be met by vehicle air conditioning systems. Heat build-up problems are expected to continue, and even be heightened, in future vehicle models as styling themes increasingly use windows to create more stylish and aerodynamic vehicles.
Greenhouse-type heating inside a glass-enclosed space occurs primarily as a result of the glass being substantially transparent to shorter wavelength, higher energy solar radiation, including visible and ultraviolet light, but substantially opaque (not transmissive) to longer wavelength, lower energy infrared radiation. Essentially, the higher energy visible and ultraviolet radiation from the sun pass through the glass windows and is then absorbed by materials inside the glassed enclosure, such as seats, upholstering, and other objects in the passenger cabin of vehicles, where the energy in the radiation is converted to heat energy. Such heat energy raises the temperature of the objects that absorb the radiation and transfers by conduction to the adjacent air, which is trapped inside the passenger cabin of the vehicle. Meanwhile, the longer wavelength, low energy infrared radiation from the sun that is transmitted by the window into the interior of passenger cabin is, for the most part, absorbed by the glass windows, which heats the glass window. Some infrared heat energy that is re-radiated from the heated seats, upholstery, and other objects in the passenger cabin may also be absorbed by the glass windows to add heat to the glass windows. Such heat energy in the glass windows is then transferred by conduction to air that is adjacent the glass surfaces, both inside and outside of the passenger cabin. Such heated air inside the passenger cabin is trapped, so heat in the passenger cabin builds up and, of course, raises the temperature inside the passenger cabin.
In the past, the transportation industry addressed heat build-up inside passenger cabins with windows that could be opened to release hot air from the interior into the atmosphere. Later, security problems as well as desire for more and faster cooling was addressed by equipping vehicles with air conditioning systems that operated on refrigeration cycle technologies that consumed large amounts of power, thus energy, to expel heat from the passenger cabins into the atmosphere and thereby cool the interiors of vehicles. Such refrigeration cycle air conditioning systems in conventional, i.e., fossil-fueled, light-duty and passenger vehicles were feasible because energy was relatively inexpensive and the power to operate these systems at peak load was only a small portion of the engine power available in the vehicles. However, because of higher energy costs, fossil fuels depletion, pollution abatement, and government regulations, vehicle manufacturers are now producing more fuel-efficient conventional vehicles with lower-powered engines for which peak conventional air conditioning loads can require as much power as the average power required for operating such vehicles. Even higher fuel efficiency vehicles as well as electric vehicles (EVs) and hybrid electric and fossil fueled vehicles (HVs) will likely be designed with even lower operating power requirements, making auxiliary loads, of which air conditioning is the largest, a much larger percentage of total energy consumption. Therefore, to meet lower energy requirements, it is desirable to reduce air conditioning loads (energy required to expel heat) in vehicles in order to reduce power consumption, size, and weight of air conditioning units needed to cool such vehicles and, in the case of EVs, to increase driving range within practical-sized on-board battery capacities. Decreasing air conditioning loads will also make it easier for vehicles to comply with the new emission regulations, such as the Supplemental Federal Test Procedure (SFTP) scheduled to be in effect by 2004 in the United States, which will require tailpipe emissions testing with air conditioning systems set at maximum operating levels.
State-of-the art methods of decreasing air-conditioning loads, i.e., power or energy needed to expel heat, have included reducing transmission of solar radiation through the windows by absorption and/or reflection of some of the radiation (see, e.g., U.S. Pat. No. 5,593,929 issued to Krumwiede et al., U.S. Pat. No. 4,943,484 issued to Goodman, U.S. Pat. No. 5,149,351 issued to Yaba et al.). However, the Federal Illuminant A Standard requires that automobile windows must transmit at least seventy percent (70%) of the visible light spectrum, and many consumers dislike heavily tinted windows in vehicles. Further, solar energy absorbed in the window glass heats the window, and a substantial amount of the window heat transfers by radiation and convection into the passenger cabin where it is not wanted. Shades or sun screens positioned adjacent inside surfaces of windows, such as those taught by Miller, U.S. Pat. No. 4,790,591, provide protection against deterioration of interior passenger cabin components, such as plastics, that are susceptible to ultraviolet light and reduces some heat build-up by reflecting ultraviolet light back through the window to the outside. However, not only do such shades and sun screens also absorb solar energy and dissipate such absorbed energy inside the passenger cabin as heat, but much of the solar radiation reflected by the shades or sun screens back to the window is also absorbed by the window and results in adding to heat build-up in the passenger cabin, as described above.
Fans have also been used to reduce solar loads on vehicles by removing hot air from the passenger cabin in two ways. First, fans with high capacities have been used to exhaust hot air quickly from the passenger cabin when the vehicle is first started. Second, lower capacity fans have been used to prevent heat build-up by running constantly to exhaust heated air from the passenger cabin continuously while the vehicle is not in use. The high capacity purge systems, however, require substantial power to operate and may require a costly redesign of the vehicle ventilation system. Due to cost, space requirements, and other considerations, these purge systems have not been widely adopted by the transportation industry, even in times of inexpensive energy availability, and they are even more impractical in vehicles designed to maximize energy efficiency. Alternative hot air exhaust systems in which lower capacity fans are operated to exhaust hot air continuously over extended periods of time, instead of immediate high capacity purging on demand. Such lower capacity fans are more energy efficient and can even be powered by solar cells and, some have even been developed as after-market accessories that can be mounted on vehicle sun roofs or on upper edges of side windows that can be rolled down or lowered slightly to provide an opening to the outside. However, convective air currents continue to carry heat conducted from solar heated windows throughout the interior of a passenger cabin, thereby resulting in a large volume of hot air filing the cabin that would have to be continually purged or exhausted in order to lower interior temperatures. Moving such large volumes of air is difficult to accomplish with lower capacity fans, even if such fans are operated continuously.
Consequently, in spite of partial successes and benefits of such efforts as selective solar radiation and absorption as well as hot air purge or exhaust systems to reduce heating effects of solar radiation on parked vehicles, and specifically to reduce cooling loads and corresponding auxiliary power load (i.e., reduce power consumption by the vehicle's air conditioning system) in order to improve fuel efficiency and reduce tailpipe emissions, none of these techniques has yet been practical or effective for maintaining interiors of parked vehicles at or near ambient temperatures on hot, sunny days while complying with federal standards for visible light transmittance and the desires of consumers for limited coloration or tint in vehicle windows.